What happens to Fusion Drive when you use Boot Camp

Now that we seem to know how to create a Fusion Drive manually, how about adding that Boot Camp partition back to your machine, so you can enjoy the new Windows 8 on your Macbook? That’s just what I attempted and here are the findings on how Boot Camp affects the partitioning of the LVM and Fusion Drive.

Here’s the layout before Boot Camp ran:

After going through the Boot Camp assistant, a look at the terminal using diskutil will give you a new layout. A new slice was created, disk1s4, reserved for Windows.

Here’s the output after:

Notice how Mac OS X is installed on disk2, which is the Logical Volume from both disk0 and disk1, aka Fusion Drive. For the Windows partition Boot Camp shrunk the CoreStorage partition on disk1s2 to make room for disk1s4, a proper non LVM partition visible to Windows. Windows will never know what’s on the CoreStorage partition and thus will not be able to let you browse your HFS+ volume from it, but at the same time it can peacefully coexist and boot from the mechanical drive, side-by-side with the Mac OS X Fusion Drive. I would even go as far to say that you could manually partition your own Boot Camp partition on disk0, i.e. shrink disk0s2 manually with diskutil on the SSD to take advantage of the faster SSD on Windows. Shrinking the LVM, by the way, is non-destructive and that’s way the Boot Camp assistant can make room for Windows without destroying the Fusion Drive.

Now, if I could only get the Windows installation to accept the GPT partition. Setup complains that the disk has an MBR layout, when it clearly isn’t. I may have to find a “creative” way of slipstreaming Windows onto that disk.


I forgot, but the way Boot Camp works is by creating a partition that looks like it resides on an MBR disk to Windows. Unfortunately, that precludes a Windows install in UEFI mode and requires a detour through the legacy BIOS CSM mode. What’s even more painfull is the fact that USB connected media won’t be accessible during boot, so if you have a Macbook Pro, like mine where you removed the optical drive and replaced it with a second hard-disk bay, you will have to remove one disk (the one without the Boot Camp partition) and install the optical drive back to initiate a Windows install. It also looked like my Win8 install wouldn’t start in CSM mode, so I had to go back to install Win7 first and then do a build-to-build upgrade to Windows 8, after having the Boot Camp drivers installed on Win7. If you don’t install the Boot Camp drivers, you may run into a blue screen during the Win8 upgrade. Anyhow, I’m back with Windows 8 installed to my mechanical drive, the same drive that is joined in a Logical Volume on the Fusion Drive LVG. The Macbook Pro is, after all, a really nice Windows PC!

  • jsspanjer

    To install windows without optical disk:
    Install windows in a virtual machine. After it copies the win image to disk it will reboot. shut down virtual machine.
    Clone with winclone. Restore this image to bootcamp partition. Windows will start with setup after booting from bootcamp partition.

    • Emeric Robert

      I tried this methode but I can’t boot from the boot camp partition. It is just not int he option list a boot. And For a reason I don’t understand, rEFIt don’t seam to work since I setup my Fusion Drive!
      How did you manage to boot from the boot camp drive to finish the installation process?

      • RobinGER

        this exact instructions did it for me :


        But before the GUIDE i separated my fusion drive into ssd and hhd again. Installed Mountain Lion only on SSD and made later the boot camp partition on the hdd. After the walkthrough it showed the Windows Partition in the Boot Menu. Then i went on creating my fusion drive again with the ssd and the rest of the hdd partition. IMPORTANT: while creating a core storage volume group, you have to select the specific partition. In my case it was disk1s2 or something. So the windows partition is NOT in that Volume Group.

        Hope this will help you out.

  • Antonio

    I am in the same situation, and opening again the iMac to disconnect the SSD it’s a pain, although if I dont find any other

    solution I’d go ahead…

    I asked the same at apple forums, but no solution yet:


    • Arthur Lee

      Have you tried getting the Boot Camp Assistant to create bootable USB installer?


      • Antonio

        Yes, I had followed those instructions too. And they worked to enable me creating the bootable USB drive, but the USB drive does not show at boot time.


        • Antonio

          I just want to give thank you.

          I followed these instructions and succeed installing bootcamp. I found a “temporary” problem that my superdrive broke just that day and I had a lot of problems to find out it was that and not something different.

          The workaround I used was extracting the DVD reader bay from a DELL laptop I had and plugged into the iMac and everthing went fine.

  • Arthur Lee

    I have a MBP 5,3 with a 500GB HDD and a 120GB SSD, with Fusion Drive setup. I ran Boot Camp Assistant to create a 100GB Boot Camp partition on my HDD.

    In turns out you can trick Boot Camp Assistant and get it to create a bootable Windows 7/8 installation USB from an ISO. I got the USB drive to boot into the installation, but I can’t get past the disk selection… (both Windows 7 and 8 installers complain). Any thoughts?

    EDIT: It’s probably the reason stated in the post. What’s the “creative way of slipstreaming Windows onto that disk”

  • Askar Bektassov

    Many thanks for your findings. I wonder if you found a possible workaround to the boot camp limitation in case of a late 2012 iMac equipped with a 3TB HDD. I feel that the users affected could: (1) resize the logical volume in order to free space on the HDD (not in excess of 2.2TB); (2) create a partition within the space so allocated (basically on the HDD only, therefore without benefits of the fusion drive); and finally (3) dedicate the newly created partition to boot camp. Since boot camp will emulate MBR exclusively on that partition, it should not complain anymore about the size of the surrounding disk.

    I am not aware of the tools available in Mac OS X to manage logical volumes, but in Linux lvm resize (shrink) is a common operation.

  • RobinGER

    I also have tried to workaround with a virtual machine, but did not succeed,
    Because the Boot Menu on my MacBook Pro wont show up any Bootable Windows 7 Drive. Wether USB or a Partition, it only shows me the two fusion drive partitions. And rEFIt wont work either. I think because the fusion drive setup wont let refit run on its boot up.

    I also tried a simple Bootcamp installation i only get like a command prompt do nothing. I think its waiting for the SuperDrive to boot up or something.

    Im very desperate and have no idea left. Can you give me some instructions on how you got a way through ?